Agent delivery systems and devices which use an expansion means can deliver a beneficial agent to an environment of use over a period of hours, days, or months. The expansion means absorbs liquid, expands, and acts to drive out beneficial agent formulation from the interior of the device in a controlled manner. Such delivery systems are well known to the prior art and include, for example, U.S. Pat. Nos. 4,077,407; 4,160,020; 4,200,098; 4,327,725; 4,595,583; 4,612,186; 4,624,945; 4,643,731; 4,663,148; 4,663,149; 4,675,174; 4,678,467; 4,684,524; 4,692,336; 4,704,118; 4,716,031; 4,717,566; 4,717,568; 4,717,718; 4,729,793; 4,772,474; 4,781,714; 4,800,056; 4,814,180; and 4,844,984.
Agent delivery devices utilizing an expansion means can be designed to deliver agent over an extended or prolonged period of time, i.e., 25-30 days or greater, and especially 60-120 days or greater. Such devices generally comprise a slowly permeable semipermeable membrane, together with a beneficial agent in a carrier which is viscous or paste-like and is extruded by the action of the expansion means over an extended time. Devices having an osmotic expandable member require a "startup" period of time for the expandable member to become activated and begin to push against the beneficial agent formulation to deliver beneficial agent to the environment of use at the desired constant rate. The startup time of the device depends upon the rate at which the semipermeable membrane allows hydration of the system and the rate at which the expansion means becomes hydrated sufficiently to begin extrusion of the beneficial formulation.
Prior art devices which have been designed to maintain continuous dosages of a beneficial agent for extended periods of time, while being extraordinarily effective for delivering beneficial agents that are hydrophilic, hydrophobic, lipophilic or lipophobic to a biological environment of use, have the disadvantage of exhibiting a significant startup time, usually of at least several days and often of up to several weeks, between administration to the subject animal or human and the onset of agent delivery at the desired rate. Therefore, it is desirable to provide means for shortening the time needed for the delivery system to start delivering a beneficial agent.